fix bad-caching of datetime with tz info at sql generation time There is a special handling for datetime with tzinfo, where value was stored in the query cache. The implementation of merge_args was simply overwriting parameters of the query with those in the query cache, expecting no collision. To fix this: * handle replacement of tzinfo in merge_args, not at sql generation time * add an assertion to ensure we've actually no collision Closes #6978316

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